1. Field of the Invention
This invention relates to a linear feed drive system for working tools, having an integrated tactile sensor system for initially positioning the tool in relation to the workpiece before the working process is started. The linear feed drive system also has an integrated weight relief.
2. The Prior Art
Mechanical, hydraulic or electric motor-driven feed drives for linear tool movements are known in the art. The function of such feed drives is to drive a working tool such as a welding torch or a spray nozzle for coating work in the direction of a workpiece, and then to guide the tool during the working process at a spacing from the workpiece that is optimal for the given working process. Controlling systems are employed for such controlled guidance in known ways. The control systems maintain the spacing desired between the tool and the workpiece via sensors that are sensitive to the spacing between the working tool and the workpiece.
In many cases, available spacing sensors are used for driving the tool from its starting position into a position spaced from the workpiece for the start of the working process. An example of such sensors are capacitive or inductive, contactlessly operating spacing sensor systems employed in thermal welding and torch cutting processes, as well as in connecting processes such as the riveting of structural components of motor vehicles. In these systems, a capacitive sensor electrode or an inductive sensor coil is connected via an electronic system with the drive of the tool so that it follows the motion of the tool from its starting position to the metallic workpiece.
The output signal of the sensor system, which is connected downstream of the capacitive electrode or inductive sensor coil, changes as the workpiece is approached. When approaching a defined adjustable nominal spacing, the feed drive is increasingly slowed down by the signal, which generally is an analog signal dependent upon the spacing, and the feed drive is then shut off under normal conditions after the nominal spacing has been reached. The tool, for example a welding or cutting torch or a rivet driving and setting tool, is subsequently driven into the defined working position. The working process starts thereafter.
However, when such capacitive or inductive sensor systems are used, the contactlessly operating sensors have a defined surface area or size, and only supply accurate voltages analog to the spacing if they face plane metallic areas on the surface of the workpiece having at least the same size. This makes it impossible to exactly position the working tool on the edges of the workpiece, or on small surfaces of the workpiece. The same problem also prevents exactly spaced guidance of the workpiece in the course of the working process if the workpiece has to be guided near material edges, or on narrowly shaped workpieces.
In most cases, particularly when metal workpieces are welded and cut, the tools are tubular or have the form of wire or nozzles, and their dimensions are very small compared to the workpieces. Therefore, the tool itself should be employed as a tactile sensor so that an exact initial position can be found on the edges and small areas of a workpiece before the working process is started. Systems are known in which the tool is slowly advanced by the linear feed drive device in the direction of the workpiece until the metallic tip of the tool, for example a torch nozzle, touches the workpiece and thereby establishes a galvanic contact between the two, which permits generating an electric switching signal to thus shut down the drive. The drawback of such systems is that the feed rate has to be greatly reduced until the tool comes into contact with the workpiece, so that the sensitive front side of the tool, e.g. a cutting nozzle, will not suffer any damage. The force generated when the tool is placed on the workpiece conforms to the following rule: EQU F=m.times.a(mass.times.acceleration)
With such feed drives, the mass acting when the tool is placed on the workpiece comprises the total mass in motion in this process, i.e., the mass of the drive motor, the mass of the linear drive and the mass of the tool, in each case based on the nozzle as the tactile sensor. In order to keep force "F" within the permissible limits, acceleration "a" has to be as low as possible.
Controlled feed drives should be as "stiff" as possible in order to keep delay times and mechanical hysteresis to a minimum. In such systems, if the "touchdown" force of the nozzle must not exceed a defined measure, the feed rate must be very low to keep the "shutdown" acceleration within limits. This requirement, however, means a substantial loss of time in the operating cycle of the machine.
In another known system, the rise in the current of the feed drive motor as the tool is put into place against the workpiece due to the higher torque as touchdown force "F" is being generated is used. Systems of this type exhibit the substantial drawback that the entire moments of inertia of the feed in this process have effects reaching into the shut-off process. In addition, sensitive interpretation of the current rise in the motor circuit is not possible because it is. necessary to rapidly control the drive in the controlled state with correspondingly rapid and intensive current increases. Consequently, low feed rates have to be employed in this case, and correspondingly high loss times in the operating cycle are unavoidable.